Quick answer
A dual battery system adds a second (auxiliary) battery that runs your fridge, lights and other accessories, while the starting battery is kept separate so you can always start the engine. When the engine is running, the auxiliary battery is charged through a voltage sensitive relay (VSR) or a DC-DC charger, and often from solar as well. You probably need one if you camp for more than a night without power and run a 12V fridge, or if you use accessories for long periods with the engine off.
Is this a DIY job?
- Difficulty: Medium for a simple under-bonnet VSR system; Pro for DC-DC, lithium, canopy or long cable runs
- Time: half a day to a full day
- Tools: heavy cable crimper, cable cutter, socket set, multimeter, drill, grommets
- Skills: high-current wiring, fusing, battery mounting
Before you wire anything, disconnect the negative terminal of each battery first, and reconnect the negatives last.
How it works
- Starting battery stays connected to the starter and vehicle electrics as normal.
- Charging device sits between the two batteries: – a VSR (isolator) links the batteries when it senses the alternator is charging, and separates them when the engine stops; or – a DC-DC charger takes power from the vehicle side and charges the auxiliary battery with a proper multi-stage charge profile. Many also accept solar input. (see DC-DC charger or VSR isolator: which do I need?)
- Auxiliary battery (usually deep-cycle AGM or lithium) powers your accessories.
- Accessories (fridge, lights, USB, inverter) are wired to the auxiliary battery, each with its own fuse.
- Solar (optional) charges the auxiliary battery through a regulator, or through a DC-DC charger with a built-in solar input.
Step by step: planning a system
- Work out your daily power use. Add up what your fridge and other accessories draw over a day, in amp-hours (Ah). The fridge maker’s data is a starting point. (see How much solar do I need for my caravan?)
- Choose the battery type and size. Deep-cycle AGM is heavier and you can only use part of its rated capacity without shortening its life. Lithium (LiFePO4) is lighter and lets you use more of its capacity, but costs more and needs a compatible charger and a cooler location.
- Choose the location. Under the bonnet (short cables, but hot), in a canopy, tray or cargo area (cooler, but longer cables), or in a caravan. Lithium batteries generally don’t suit hot engine bays: heat shortens their life, and some makers say not to fit them there at all.
- Choose the charging device. A VSR suits matching lead-acid batteries close together on vehicles with a conventional alternator. A DC-DC charger suits smart alternators, lithium batteries, long cable runs and solar.
- Size the cables and fuses. The link cable between the batteries carries high current and needs to be heavy, with a fuse or circuit breaker near each battery. (see What size cable do I need for my 12V accessory?)
- Mount the battery securely. Use a proper tray or box with a hold-down that won’t let the battery move in a crash or on corrugations.
- Wire accessories to the auxiliary battery with individual fuses, ideally through a fuse block, and a low-voltage cut-off where needed.
- Test the system. With the engine running, check the auxiliary battery is charging. With the engine off, check accessories run from the auxiliary battery and the starting battery stays isolated.
Specs table
VSR vs DC-DC at a glance
| VSR (isolator) | DC-DC charger | |
|---|---|---|
| How it charges | Connects auxiliary battery straight to the vehicle system when charging | Converts vehicle power into a multi-stage charge for the auxiliary battery |
| Smart (variable-voltage) alternators | Often won’t fully charge the auxiliary battery | Designed to cope |
| Lithium auxiliary battery | Generally not suitable | Common choice (with a lithium charge profile) |
| Long cable runs (canopy, caravan) | Voltage drop reduces charging | Compensates better |
| Solar input | Needs a separate regulator | Many have one built in |
| Cost | Lower | Higher |
What you’ll need
- deep cycle battery AGM or lithium battery 12V
- voltage sensitive relay or DC-DC charger
- battery tray or battery box
- heavy duty single core auto cable / twin core auto cable
- circuit breaker or MIDI fuse holder (one near each battery)
- cable lugs and adhesive heat shrink
- fuse block
- battery monitor (optional)
When to call an auto electrician
- Your vehicle has a smart alternator, stop-start or a battery management system.
- You’re fitting a lithium battery or a battery in a canopy, tray or caravan.
- You need to run heavy cable through the firewall or along the chassis.
- You’re adding an inverter or 240V outlets. Fixed 240V wiring needs a licensed electrician. (see How do I choose and install an inverter?)
- The auxiliary battery isn’t charging, or the starting battery goes flat when camping.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: Can I just run my fridge off the starting battery? A: You can, but you risk a flat battery and a car that won’t start. A dual battery system or a portable power station keeps the starting battery for starting.
Q: Do I need a DC-DC charger or will a VSR do? A: If you have a smart alternator, a lithium auxiliary battery, a long cable run or want solar input, a DC-DC charger is usually the better choice. A VSR can suit older vehicles with matching lead-acid batteries close together. (see DC-DC charger or VSR isolator: which do I need?)
Q: Can I put the second battery in the cabin? A: Only a battery designed for it, in a sealed or vented box, securely restrained. Flooded batteries vent gas and shouldn’t be in the cabin. Many lithium and AGM batteries are sealed, but follow the maker’s instructions.
Q: What size auxiliary battery do I need? A: Work out your daily use, then allow for how many days without charging you want and how much of the battery’s capacity you can use. Lithium usually lets you use more of its rating than AGM.
Sources
- REDARC, “The truth about alternators” – smart alternators vary their output; a simple isolator won’t charge an auxiliary battery properly from one; DC-DC charger recommended; solar and vehicle charging together. https://redarcelectronics.com/blogs/blog/the-truth-about-alternators (checked October 2026)
- REDARC, “BCDC v DCDC dual battery chargers” – vehicles with variable-voltage alternators need a DC-DC charger; lithium charge profiles; solar input; isolator suits traditional alternators. https://www.redarcelectronics.com/au/discover/bcdc-v-dcdc-dual-battery-chargers/ (checked October 2026)
- REDARC, “The REDARC call log: lithium battery FAQs” – lithium batteries aren’t designed for engine bays; near-full usable capacity compared with AGM. https://www.redarcelectronics.com/au/discover/the-redarc-call-log-lithium-battery-faqs/ (checked October 2026)
- Solar4RVs, “Understanding cable size types: mm, mm², B&S, AWG” – cable sizing for DC-DC chargers and battery links. https://www.solar4rvs.com.au/buying/buyer-guides/understanding-cable-size-types-mm-mm2-b_s-awg/ (checked October 2026)